Hard Doom is a replacement/enhancement mod that replaces all monsters in Doom 2 with versions that are similar in role, but more dangerous. In general, Hard Doom is geared towards players who want a more difficult, genuine survival experience, and it has a variety of visual effects that are more subtle and don't block your view of the action. Over the years since its initial release, it has been meticulously balanced and rebalanced, with new features and effects added to make this a great mod to play through the popular megawads with, without changing Doom's flavor or core gameplay. This mod requires ZDoom, GZDoom, or Zandronum in order to play!
This mod has come a far way. Is it perfect? No, but it does its thing very well.
I was there the first hour when the mod was released and really liked how it put a twist on old maps with a new difficulty. OFten I was criticising this mod for bein too ill-polished.
But that was a long time ago and I think all the additions, ranging from more difficult monsters to more weapons really improve the gameplay as a whole and is just much more fun.